1. Field of the Invention
This invention relates to a time calibration method for a security system using sensors that enables multiple devices constituting the security system to share accurate time data.
2. Background Art
Nowadays, security surveillance systems for monitoring security zones demarcated in shops, stores, financial institutions, offices and other such locations are ubiquitous. In the typical system, the monitored site is equipped with suitable window sensors, door sensors, glass shatter sensors, infrared (body heat) sensors and/or various other sensing devices for detecting the intrusion of unauthorized persons and with means responsive to the outputs of these sensors for sending specified security information to a security monitoring center via a telephone line or the like. The security surveillance center uses the security information to monitor intrusion of unauthorized persons.
Japanese Unexamined Patent Publication No. 1-131997, for example, teaches a security surveillance system of this type that is equipped with a time data generator and a memory for storing sensor operation data in association with time data so as to facilitate later analysis of the intruder's entry and escape routes by recreating the signals output by the sensors at the time of the incident.
Although the time data generator is usually a comparatively accurate instrument utilizing a crystal oscillator, its accuracy is nevertheless limited to a monthly deviation of several seconds to several tens of seconds. Unless appropriately calibrated, therefore, errors measured in seconds will arise between the time data associated with the sensor operation data and the time data associated with security surveillance center data and/or the time data of the different surveillance devices installed in the same security zone.
When such a situation arises in which the various devices making up the security surveillance center do not share identical time data, analysis of the operation data of the intrusion sensors installed in the security zone is liable not to provide adequate information on the intruder's entry route and/or escape route because only a rough comparison can be made between the intrusion sensor operation data and the time data associated with the security surveillance center data.
Security surveillance systems of the type under discussion are often installed in association with an automatic business hour (opening/closing time) system. This system should desirably be highly time-accurate, particularly when business hours are prescribed by law as is sometimes the case with financial institutions. When only crystal oscillator clocks are used, the time error eventually accumulates to the point where accurate control of business hours becomes impossible.
Moreover, when the zone under surveillance is extensive, as in the case of a large retail outlet or factory, multiple security control units are installed in the same building in order to accommodate the large number of intrusion sensors required, and a separate time data generator is provided in each control unit. In such a case, situations may arise in which analysis of intrusion sensor operation becomes impossible owing to deviation among the time data of the time data generators in the different security control units. For instance, later analysis of an intrusion becomes extremely difficult when a number of intruders make a coordinated entry through different windows and doors.
A similar problem arises when simultaneous break-ins occur at the premises of two or more tenants who occupy the same building and are separately equipped with their own security control units.
The conventional practice has therefore been to adopt some manual or automated method of calibrating the clocks at the surveillance sites. The manual method generally involves periodic visits by center personnel to calibrate the individual security control units using accurately calibrated clocks or time signals. In the automatic method, time data from a master clock at the security surveillance center is sent to the surveillance system at each monitored site using a surveillance telecommunications circuit and the individual surveillance systems use the received time data to calibrate the clocks of their surveillance devices.
The former method is high in cost, however, because it requires a considerable amount of human labor for the calibration. In addition, time data from the security surveillance center and that from the security surveillance equipment at the individual surveillance sites often do not match perfectly, so that the accuracy of the time analysis of the surveillance data unavoidably remains low.
While the latter method does not suffer any particular problem regarding time analysis so long as the accuracy of the security surveillance center master clock is sufficiently high, the calibration requires a great amount of time when the number of surveillance sites reaches several thousand or more. As such, it is likely to adversely affect security and surveillance operations. Although this disadvantage can be avoided by using a large, high-performance computer system, this leads to another problem of high hardware costs.
A particularly critical situation arises when it becomes necessary to simultaneously calibrate every clock throughout the system at a specific time, such as when adjusting to a seasonal time change or a leap second.